1. Field of the Invention
The present invention relates to an oxygen concentration detection apparatus for detecting the oxygen concentration in the exhaust gas discharged from an internal combustion engine.
2. Description of the Related Art
As means for purifying exhaust gas of the engine, improving fuel economy and the like, there is known an air-fuel ratio controller which detects the oxygen concentration in the exhaust gas and controls in a feedback manner the air-fuel ratio of a mixture, which is supplied to the engine, to a required air-fuel ratio in accordance with the detected oxygen concentration level. In an oxygen concentration detection apparatus used in such a air-fuel ratio controller, as disclosed, for example, in Japanese Patent Laid-Open No. 58-1531, there is known an oxygen concentration detection apparatus which generates an output in proportion to the oxygen concentration in the exhaust gas in a region where the air-fuel ratio of a mixture supplied to the engine is greater than a stoichiometric air-fuel ratio. As disclosed in Japanese Patent Laid-Open No. 62-76446, there is known a controller for such an oxygen concentration detection apparatus in which the current supply to an oxygen pumping device is delayed by a predetermined length of time from the start of current supply to prevent blackening of the oxygen concentration detection apparatus and incorrect detections due to a delayed response of a heating element which heats the oxygen pumping device. The term "blackening" means that oxygen ions are removed from the solid electrolytic member. There is also known an oxygen concentration detection apparatus, as disclosed in Japanese Patent Laid-Open No. 62-203950, in which after the engine has been started, the current supplied to the heating element is decreased until a predetermined time has elapsed to prevent thermal shock which would otherwise result from a sudden heating of the oxygen pumping device and might damage the oxygen concentration detection apparatus.
In the aforementioned conventional oxygen concentration detection apparatus, the predetermined length of time is generally set to be longer than the required time for ordinary conditions of use (for example, at the outside air temperature of 20 degrees or therearound) to prevent problems from occurring even when the most unfavorable air temperature condition for automobile use (for example, when starting the engine at an outside air temperature of -40 degrees) occurs.
For this reason, under ordinary conditions of use, in the conventional oxygen concentration detection apparatus, even when feedback control can be performed after current supply to the oxygen pumping device has been started, current cannot be supplied to the oxygen pumping device until the aforementioned predetermined length of time has elapsed. Therefore, during the predetermined length of time, since the ratio of the air-fuel mixture cannot be controlled by a feedback control method, it is controlled by an open loop control method.
In addition, under ordinary conditions of use, in the conventional oxygen concentration detection apparatuses, even when current can be supplied to the heating element without decreasing the magnitude of current supplied after the engine has been started, feedback control cannot be performed until the predetermined length of time has elapsed. Therefore, during this length of time, the ratio of the air-fuel mixture is controlled by the open loop control method.
Further, in the conventional oxygen concentration detection apparatus, optimal air-fuel ratio control cannot be started every time in the shortest period of time, which gives rise to such problems as insufficient purification of the exhaust gas and reduced fuel economy.